Surface acoustic wave devices are circuit devices which conduct signal processing of converting electric signals to surface acoustic waves, and they are used in filters, resonators, delay lines, etc. Usually electrodes of conducting film of, e.g., metal, which are called inter-digital transducers (IDTs) are formed on a piezoelectric substrate for converting electric signals to surface acoustic waves, and vice versa.
Characteristics of the surface acoustic waves depend on propagation characteristics thereof on the piezoelectric substrates. Especially for higher frequencies of the surface acoustic wave devices, the piezoelectric substrates must have high propagation velocities for the surface acoustic waves.
As substrate materials for surface acoustic waves, quartz, lithium tantalate (LiTaO.sub.3), lithium niobate (LiNbO.sub.3), lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7), etc., are conventionally known. As surface acoustic waves to be used in the surface acoustic wave devices, Rayleigh waves and leaky surface waves are mainly known.
Rayleigh waves are surface acoustic waves which propagate on the surfaces of elastic bodies, and propagate without radiating their energy into the elastic bodies, i.e., without theoretical propagation losses. As substrate materials to be used in the surface acoustic wave devices using Rayleigh waves, an ST-cut quartz with a 3100 m/sec propagation velocity, an X-cut 112.degree.-Y propagating LiTaO.sub.3 of a 3300 m/sec propagation velocity, 128.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4000 m/sec propagation velocity, a 45.degree.-rotated X-cut Z-propagating Li.sub.2 B.sub.4 O.sub.7 with a 3400 propagation velocity, etc., are known.
On the other hand, the use of leaky surface waves in the surface acoustic wave devices have been studied. The leaky surface waves are surface acoustic waves which propagate on the surfaces of elastic bodies, radiating one transverse component thereof as bulk waves in the direction of depth of the elastic bodies.
Generally, leaky surface waves have such large propagation losses due to the above radiation that it is difficult to use leaky surface waves in the surface acoustic wave devices. At a certain cut angle and in a certain propagation direction, however, usually the propagation losses are relatively small, thus leaky surface waves could be used in the surface acoustic wave devices.
Leaky surface waves, which have higher propagation velocity than Rayleigh waves, are widely used in surface acoustic wave devices for relatively high frequencies (above the UHF band).
As substrate materials for the surface acoustic wave devices using leaky surface waves, an LST-cut quartz with a 3900 m/sec propagation velocity, a 36.degree.-rotated Y-cut X-propagating LiTaO.sub.3 with a 4200 m/sec propagation velocity, a 41.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4500 m/sec propagation velocity, a 64.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4500 m/sec propagation velocity, etc., are known.
The inventors of the present application recently further developed the theory of leaky surface waves and ascertained the presence of surface waves (hereinafter called longitudinal leaky waves) which propagate with the longitudinal waves as the main component, radiating two transverse wave components as bulk waves into substrates.
They ascertained the presence of surface acoustic waves of a propagation velocity as high as 5000-7500 m/sec on lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7) whose cut angles and propagation directions are an Eulerian angle representation of (0.degree.-45.degree., 30.degree.-90.degree., 40.degree.-90.degree.) and a direction equivalent to the representation (see Japanese Patent Laid-Open Publication No. 112763/1994).
It is known that especially, in a case that cut angles and propagation directions are an Eulerian angle representation of (0.degree.-45.degree., 38.degree.-55.degree., 80.degree.-90.degree.) and a direction equivalent to the representation, the surface acoustic waves not only have a propagation velocity as high as 6500-7200 m/sec, but also exhibit a large electromechanical coupling coefficient (k.sup.2) and good propagation characteristics having low propagation loss.
It is possible to apply leaky surface waves and longitudinal leaky surface waves, whose propagation velocity is, as described above, generally higher than Rayleigh waves, which are applicable to surface acoustic wave devices of high frequencies.
It is known that to use leaky surface waves and longitudinal leaky surface waves in surface acoustic wave devices, at least one IDT is necessary, and propagation characteristics vary greatly with thicknesses of the IDT. That is, the IDT is usually formed of a metal film which comprises aluminium (Al) as a main component, and it is known that characteristics of the surface acoustic waves vary depending on thicknesses (normalized film thickness h/.lambda.) of the conducting film normalized by wavelengths of the surface acoustic waves.
Because a conducting film of, e.g., Al, abruptly increases the resistivity as its thickness increases, it is general that the conducting film is set at a film thickness of above 1000 .ANG. which permits the electric resistance to be decreased Accordingly this sets a lower threshold of values of the normalized film thickness (h/.lambda.) which can sufficiently decrease the electric resistance As the surface acoustic wave devices have higher frequencies, the surface acoustic waves have shorter wavelengths. A lower threshold value of a required normalized film thickness is increased up to above 1-2%.
For example, in a case that 1 GHz frequency signals are processed by using leaky surface waves and longitudinal leaky surface waves, a required normalized film thickness is above 2.5% for LST cut crystal, above 2.3% for 36.degree. Y-cut X-propagation LiTaO.sub.3, above 2.2% for 41.degree. Y-cut X-propagation LiNbO.sub.3, and above 1.5% for Li.sub.2 B.sub.4 O.sub.7 of cut angles and propagation directions of an Eulerian angle representation of (0.degree., 47.3.degree., 90.degree.). For high frequencies, the normalized film thickness has larger lower threshold values.
The inventors of the present application, however, conducted a numerical simulation of dispersion characteristics of leaky surface waves and longitudinal leaky surface waves and obtained a result that propagation losses of the leaky surface waves and the longitudinal leaky surface waves abruptly increase when the metal film on the substrate surface is thicker than a normalized film thickness.
Next, the computing method for the simulation and the result of the computation will be explained.
The numerical simulation was conducted on a single electrode which is usually used as an IDT. FIG. 1 shows the computation model. Electrodes (strips) 12 are formed on a piezoelectric substrate 10 at a pitch P in the direction of propagation of surface acoustic waves. A electrode width is represented by M, and a film thickness is represented by Hm. A direction of propagation of surface acoustic, waves on the surface of the piezoelectric substrate 10 is represented by X1, a direction of depth of the piezoelectric substrate 10 is represented by X3, and directions perpendicular to the directions X1; X3 are respectively represented by X2. The propagation characteristics of the surface acoustic waves of the IDT generate first Bragg reflection by the periodic perturbation effect of the electrodes and frequency dispersion of the propagation coefficient .kappa. (wave number). First, the frequency dispersion of the propagation coefficient .kappa. will be computed. A displacement Ui of surface acoustic waves and an electrostatic potential .PHI. are expressed by an addition of space harmonics given by the following formulas 1 to 4, using Floquet's theorem. ##EQU1##
Here, an attenuation coefficient .alpha..sup.(m,n) for the direction X3 of propagation of surface acoustic waves and an amplitude coefficient .beta.i.sup.(m,n) are given by setting a propagation coefficient .kappa. and an angular frequency .omega. and by solving for m in the respective regions the equation of motion given by Formula 5 and Maxwell's equations of Formula 6 on quasielectrostatic approximation.
Formula 5 EQU Cijkl Uk,li+ekij.phi.,ki=.rho.Uj
Formula 6 EQU eikl Uk,li+.epsilon.ik.phi.,ki=0 (i,j,k,l=1,2,3)
where Cijkl, ekij, and .epsilon.ik respectively represent tensors of an elastic coefficient, a piezoelectric coefficient and a dielectric constant, and .phi. represents a density
An amplitude constant A.sup.(m,n) of the space harmonics is given by substituting Formulas 1 to 4 into boundary conditions As mechanical boundary conditions, displacements U1, U2, U3 are continuous below the electrodes, stresses T31, T32, T33 are continuous below the electrodes, and the stresses T31, T32, T33 are 0 between the electrodes As electric boundary conditions, the potential .PHI. is constant below the electrodes, a boundary of the potential .phi. and an electric flux density, and a perpendicular component D3 are continuous between the electrodes
In a shorted strip, in which electrodes are shorted with each other, a potential .PHI. on the strips must be 0. In an open strip, in which electrodes are not shorted with each other, total charges on the electrodes must be 0.
A propagation coefficient .kappa. for an angular frequency .omega. can be given by the above computation. The computation was made with a sufficiently high order m of the space harmonics.
Generally, a periodic perturbation due to the electrodes generates a stop band in which a propagation coefficient satisfies the first Bragg's reflection condition (Re(.kappa.)=.pi./P). Frequencies on both edges of a stop band of the short strip are represented respectively by fs1 and fs2, and frequencies on both edges of a stop band of the open strip are represented respectively by fo1 and fo2. Imaginary components of propagation coefficients .kappa. are 0 (Im(.kappa.)=0) for Rayleigh waves, which, in computation, generate no propagation loss, but those for leaky surface waves and longitudinal leaky surface waves are not 0 (Im(.kappa.).notident.0).
Based on frequencies on the edges of stop bands of the short strip and the open strip, parameters necessary for a Smith's cross-field model, which is widely used as a surface acoustic wave device design method, can be extracted. As the parameters were given an acoustic impedance mismatch .epsilon.(.epsilon.=(Zo/Zm)-1, where Zo represents an acoustic impedance of parts having no strips, and Zm represents an acoustic impedance of parts having strips), and a susceptance Be indicating an energy storage effect and an electromechanical coupling factor k.sup.2.
In a case that an array of strips is bidirectional, frequencies agree with each other at either of stop band edges of the short strip and the open strip, but do not agree, in a case that the array of strips is unidirectional (E. L. Adler et al, "Arbitrarily Oriented SAW Gratings: Network Model and the Coupling-of-Modes Description", IEEE trans. on Ultrason. Ferroelec. Freq. Contr., vol. UFFC-38, no. 3, pp. 220-230 (1991)). In this case, a susceptance Be indicating the unidirection was also given. Propagation loss .alpha.s1; .alpha.s2 at the stop band edges fs1; fs2 of the short strip, and propagation losses .alpha.o1; .alpha.o2 at the stop band edges fo1; fo2 were taken from a value of the propagation loss used in the Smith's cross-field model, where the wave number in the model coincides with the wave number obtained from the given dispersion curves.
Next, as one example of the specific computation, FIGS. 2 to 5 show computed values of dependence of dispersion characteristics of Li.sub.2 B.sub.4 O.sub.7 of cut angles and propagation directions of an Eulerian angle representation of (0.degree., 47.30.degree., 90.degree.) with respect to longitudinal leaky surface waves on the Al film thickness.
FIGS. 2 to 5 respectively show propagation losses .alpha.s1, .alpha.s2 at the stop band edges, electromechanical coupling factors k.sup.2, susceptances Be which are indicative of energy storage effects, and acoustic impedance mismatches .epsilon.. The susceptances Be are normalized by the acoustic admittances (inverse numbers of acoustic impedances Zo). Here, a temperature was 25.degree. C., a strip width (M/P) normalized by a strip pitch P was 0.5, and the strips were formed of aluminium. Based on these values it is shown that a propagation loss is above 0.04 dB/.lambda. when a normalized aluminium film thickness which is normalized by a value 2P twice the strip pitch is above 1.7%. Thus, it is found that it is difficult to design a surface acoustic wave device which is operative at relatively high frequencies when the Al film is set at a normalized film thickness of a sufficiently small electric resistance.
Similarly, also with respect to other leaky surface waves, the propagation loss is large at normalized film thicknesses which are 1.5-4.0%.
As described above, in using surface acoustic waves, such as longitudinal leaky surface waves, which propagate, radiating bulk waves in the direction of depth of a piezoelectric substrate, since the propagation loss increases when a normalized film thickness of the IDT exceeds a certain extent, the propagation loss is large at a sufficiently low internal electric resistance of the electrodes, and resultantly insertion losses of surface acoustic wave devices are largered.
As described above, a surface acoustic wave device, which converts electric signals to surface acoustic waves propagating on the surface of a piezoelectric substrate, and vice versa, by the use of an IDT (inter-digital transducer) provided on the piezoelectric substrate, functions as a filter, resonator, delay lines, etc. The typical structure of the IDT comprises, as shown in FIG. 6A, a plurality of pairs of electrodes 12 in the form of metal strips which are arranged on a piezoelectric substrate 10. The electrodes 12 have a thickness necessary to ensure low internal electric resistance in the IDT.
As in a case, however, that a normalized film thickness h/.lambda. normalized by an IDT pitch is above 1%, when the metal strips are relatively thick, reflection of the surface waves on the boundary portions of the metal strips is a problem. This reflection increases deformation of frequency response, which hinders the intended characteristics.
As a structure of the IDT included in the conventional surface acoustic wave device, there is a known structure whose sectional view is shown in FIG. 6B (see Japanese Patent Publication No. Tokkyosho 56-36604/1978)
In the IDT structure shown in FIG. 6B, recesses 14 are in advance formed in regions of a piezoelectric substrate 10 where electrodes 12 are to be formed, and the electrodes 12 are formed in the recesses 14. Because of the piezoelectric substrate 10 present between the side surfaces of the electrodes 12, mass discontinuity can be decreased. This conventional technique intends to reduce the effect of the reflection for improved characteristics Propagation characteristics of the surface waves vary with depths of the recesses 14, and it is necessary to accurately control the depth of the recesses 14. The piezoelectric substrate 10 can be much damaged depending on the method for forming the recesses 12, and required piezoelectric characteristics can not often be obtained.
According to the invention of Japanese Patent Publication No. Tokkosho 56-36604/1981, the electrode portions are disposed on the recess bottoms, and the depth of the recesses is so set that acoustic impedances of surface acoustic waves at the electrode portions and the portions without the electrodes are substantially equal to each other, whereby reflection on the electrodes is reduced.
As a structure of the IDT included in the conventional surface acoustic wave device, there is known a structure whose sectional view is shown in FIG. 6C.
In the IDT structure shown in FIG. 6C, a mask which is opened except regions thereof where electrodes 12 are to be formed in is formed on an aluminium metal layer formed on the entire surface, and parts of the aluminium layer in the opened regions are anode-oxidized to form anode oxide films 16 between the side surfaces of the electrodes 12. Because of masses of the anode oxide films 16 between the electrodes 12, mass discontinuity can be decreased (see Japanese Patent Publication No. Tokkyosho 59-8964/1984) A thickness of the anode oxide film 16, however, is defined by a thickness of the aluminium metal layer forming the electrodes 12, and a metal material forming the electrodes 12 and a thickness of the electrodes 12 are also limited by the fabrication process.
Thus, it has been conventionally difficult to fabricate IDTs of structures which can decrease the mass discontinuity of the IDTs.
An object of the present invention is to provide a surface acoustic wave device which uses surface acoustic waves, such as longitudinal leaky surface waves, which propagate, radiating bulk waves in the direction of depth of a piezoelectric substrate, and which can signal process relatively high frequencies without increasing propagation losses and with sufficiently low electric resistance.
Another object of the present invention is to provide a method for fabricating a surface acoustic wave device, which can decrease mass discontinuity at the electrodes, causes no deformation of frequency response waveforms, and can effectively prevent degradation of propagation characteristics.